For anyone wanting to create 3D overlays, there are lots of different ways to do it! Aurasma needs 3D assets to be in DAE format - and more specifically, it works best with the OpenCollada DAE format.
A number of 3D applications can produce dae files - Aurasma was built specifically to read dae files encoded using the OpenCollada export format. Other formats such as the Autodesk Collada dae format are occasionally passable (depending on the complexity of your 3d asset). So far the OpenCollada group has only released exporter plugins for Autodesk 3ds Max and Maya - we highly recommend you use one of these programs if you can.
Other such 3D applications, like Cinema 4D, Google Sketchup, Houdini, Solidworks, Rhino, and Apps like 123D, are able to produce DAE files - or a file which can be converted into a DAE. These DAE files, however, are so far removed from the OpenCollada format Aurasma was designed to read that it isn't able to process them.
Tests have shown using Blender to export dae files works successfully. It is entirely possible that you may be able to build your assets in one 3D program and then export them to Blender (using an FBX format) where you can export them again as a dae. Note though this will not always be successful.
Blender is a fantastic FREE bit of 3D software – it’s very easy to use and there are tonnes of tutorials out there on it.
You can download the software here: http://www.blender.org/download/
And find tutorials here: http://www.blender.org/support/tutorials/